Automatic control method for supplying anaesthetic to a low flow-type closed circuit

ABSTRACT

The method comprises the stages of a) setting the ventilatory rate (VR) for the patient; b) automatically setting the flow of fresh gas (FFG) to 10% of the VR; c) determining the volatile anaesthetic fractions inhaled (% IF) and exhaled (% EF); d) opening the dial of the anaesthetic vaporiser to the value resulting from multiplying the differential (% IF−% EF) by 10; e) supplying the circuit with a quantity of anaesthetic that covers, at least, the total quantity of anaesthetic consumed by the patient, by means of opening the dial of the anaesthetic vaporiser; and f) mixing the quantity of anaesthetic with the FFG before introducing the mixture into the patient. The method has application in the administration of inhaled anaesthetics in low-flow anaesthetic systems.

FIELD OF THE INVENTION

The invention relates to the automatic control of supply of volatileanaesthetic to a closed circuit at low flow rates.

BACKGROUND OF THE INVENTION

A closed circuit of anaesthetic is an anaesthetic system based on there-administration of exhaled gases, from which carbon dioxide has beenabsorbed, to which oxygen is added and the anaesthetics consumed.

As is known, the concentration of volatile anaesthetic in an anaestheticcircuit is determined by the anaesthetist as he or she wishes and inaccordance with the Minimum Alveolar Concentration (M.A.C.) for eachanaesthetic.

Changes to this M.A.C. are those which, classically, in a circuit of lowflow, are modified by varying the flow of fresh gas and, therefore, theamount of volatile anaesthetic dragged along by the current of freshgas. This way of controlling the anaesthetic presents some difficulties,among which the excessive consumption of anaesthetic can be mentioned.

Now it has been found that the concentration of anaesthetic canadvantageously be regulated by modifying the dial of the anaestheticvaporiser instead of modifying the flow of fresh gas. This way ofcontrolling the concentration of anaesthetic has, among otheradvantages, that of reducing the consumption of anaesthetic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an initial phase of filling the closed circuit.

FIG. 2 illustrates an inhaling phase.

FIG. 3 illustrates an exhaling phase.

DESCRIPTION OF THE REFERENCE NUMERALS

1. Gas analyser

2. Mechanic ventilator.

3. Reservoir ball or bag for manual ventilation

4. Concertina

5. Patient's lungs

6. Inhaling branch unidirectional valve

7. Exhaling branch unidirectional valve

8. Adjustable valve for expulsing excess gas or “pop-off”.

9. Canister

10. O₂ intake

11. Air intake

12. Vaporiser

13. Dial of the vaporiser

14. Rotameters

15. Inhaling branch

16. Exhaling branch

17. “Y” or “T” junction.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a method of automatic control for the supplyof a volatile anaesthetic to a closed circuit at low flow rates.

The automatic control method for the supply of a volatile anaesthetic toa closed anaesthetic circuit, object of this invention, that comprisesthe stage of mixing the fresh gas and the volatile anaesthetic beforebeing introduced into the patient, is characterised because:

a) the ventilatory rate of the patient is set;

b) the flow of fresh gas is set automatically to 10% of the setventilatory rate;

c) the fraction inhaled (% IF) and exhaled (% EF) by the patient ofvolatile anaesthetic are determined;

d) the dial of the anaesthetic vaporiser is set to the value resultingfrom multiplying by 10 the differential of the fraction inhaled (% IF)and fraction exhaled (% EF) of volatile anaesthetic by the patient, thatis to say, % dial=(% IF−% EF)×10, changing, in the same proportion, theopening of said dial every time that said differential of inhaledfraction (% IF) and exhaled fraction (% EF) of volatile anaesthetic bythe patient varies

e) a quantity of anaesthetic is supplied to the circuit that covers, atleast, the total amount of anaesthetic consumed by the patient, by meansof the opening of the anaesthetic vaporiser dial; and

f) the quantity of anaesthetic is mixed with the flow of fresh gasbefore being introduced into the patient.

In the sense used in this description, a closed circuit of anaestheticat low flows refers to an anaesthetic system based on there-administration of the exhaled gases, from which the carbon dioxidehas been absorbed, and to which oxygen and the anaesthetics consumed areadded, and in which the total flow of fresh gas (oxygen) is less than 4l/min.

A typical closed anaesthetic circuit is illustrated in FIGS. 1-3,designed for working with volatile anaesthetics and which can operate atlow flow rates, in which the method of automatic control of volatileanaesthetic, object of this invention, can be applied, is comprised ofthe following elements: source of fresh gas 14;

a source of volatile anaesthetic that contains a vaporiser 12 thatincorporates a dial 13 for regulating the anaesthetic output, in such away that the anaesthetic is vaporised before being mixed and draggedalong by the fresh gas;

an input of fresh gas carrying the volatile anaesthetic;

two tubes that constitute the inhaling and exhaling branches 15, 16respectively of the circuit;

two unidirectional valves, one of which 6 is on the inhaling branch 15and the other 7 is on the exhaling branch 16, that determine thedirection of flow within the circuit;

an adjustable valve 8 for expulsing excess gas;

a piece 17 in the form of a “Y” or a “T” for connection to the aerialpathway of the patient;

a reservoir ball or bag 3, with capacity greater than the flow volume,adaptable or which can be substituted by the concertina 4 of aventilator 2; and

a canister or carbon dioxide absorbing device 9.

While exhaling (see FIG. 3), the inhaling valve 6 is closed and theexhaled gases are transferred to the balloon, where the fresh gaseswhich are entering the system at this time also arrive, the excess ofgas from the patient is expelled outside via the adjustable valve forexpulsion of excess gas 8.

While inhaling (see FIG. 2), the exhaling valve 7 is closed, theinhaling valve 6 is opened and the patient receives the fresh gas thatis entering in the circuit at this time, the fresh gas that occupied thecanister 9 during exhaling and the exhaled gas contained in the balloonwhich is stripped of its carbon dioxide as the gas passes through thecanister 9.

The term “patient”, as it is used in this description, includes anysubject, person or animal, who receives volatile anaesthetic.

As volatile anaesthetic any known inhaled volatile anaesthetics can beused, for example, halogenated inhaled anaesthetics such as halothane,desflurane, isoflurane and sevoflurane.

The method of the invention starts by setting the ventilatory rate (VR)to be breathed by the patient. This is the quantity of air that entersand leaves the lungs of the patient in one minute, and is set byadjusting the ventilator to the desired ventilatory rate. The VR isequal to the sum of the alveolar volume [part of the VR that arrives atthe alveoli and serves for hematosis or gas exchange (oxygen, carbondioxide and anaethetic)] and the dead volume [quantity of air thatenters and leaves the lungs without reaching the pulmonary alveoli, thatis to say, without gas exchange].

Once the VR has been established, the flow of fresh gas is automaticallyset. The fresh gas consists only of oxygen and serves as a vehicle forintroducing the volatile anaesthetic, in vapour form, into the circuitand then into the patient. The flow of fresh gas has to cover the needsof the oxygen consumed, that is to say, the quantity of oxygen necessaryfor maintaining the basal metabolic consumption. In general, said oxygenconsumption can be calculated from the formula [1]:

Oxygen consumption=weight (kg)^(3/4)×10  [1]

The result obtained from applying formula [1] normally ranges between200 and 300 ml/min, which means a supply of 500 ml/min of oxygen to aclosed anaesthetic circuit. This circuit normally has a capacity of 4 or5 litres, filled with oxygen at 100%, thus guaranteeing that any oxygenconsumption needs are covered.

In the method of the present invention, the flow of fresh gas (FFG) isset automatically to 10% of the established ventilatory rate (VR), thatis to say, FFG=10% X VR.

The fraction of anaesthetic inhaled by the patient (% IF) is thepercentage of the volatile anaesthetic that the ventilator administerson each inhalation. This can be determined by a suitable piece ofequipment using any conventional technique appropriate for quantifyingthe gases that make up the fraction inhaled by the patient.

The fraction exhaled by the anaesthetised patient (% EF) is thepercentage of volatile anaesthetic that is measured in the exhalingbranch of the circuit, that is to say, the quantity of anaesthetic thatleaves the lung of the patient once a certain quantity of the inhaledfraction has been captured by the anaesthetised patient (% IF). Thefraction exhaled by the anaesthetised patient can be determined by meansof suitable equipment using any conventional technique suitable fordetermining and quantifying the gases that constitute the fractionexhaled by the patient, including the carbon dioxide levels.

Next, in accordance with the method of this invention, the dial of thevaporiser of the volatile anaesthetic is opened to the valuecorresponding to multiplying by 10 the differential of the fractionsinhaled (% IF) and exhaled (% EF) by the anaesthetised patient, that isto say, the dial opening corresponds to the result of carrying out theoperation (% IF−% EF)×10, thus establishing the equilibrium that isshown in equation [4] (see later).

Every time that the differential (% IF−% EF) varies in time, forexample, due to variations in capture of anaesthetic by the patient, thevaporiser dial should changed in exactly in this same direction suchthat the concentration pre-established by the anaesthetist (M.A.C.) doesnot vary in time (dynamic stability of the circuit).

Next, by means of opening the dial of the anaesthetic dial a quantity ofvolatile anaesthetic is supplied to the low-flow closed anaestheticcircuit that covers, at least, the total quantity of anaestheticconsumed by the patient, a quantity that is determined by equation [2]:

TQAC=(% IF−% EF)×V _(ALV)  [2]

Where

TQAC is the total quantity of anaesthetic consumed by the patient;

% IF is the fraction inhaled by the anaesthetised patient;

% EF is the fraction exhaled by the anaesthetised patient; and

V_(ALV) is the alveolar volume of ventilation.

Said quantity of anaesthetic consumed by the patient is derived from theamount of anaesthetic supplied to the circuit that, in a particularembodiment of the invention, can be determined by equation [3]:

QASC=FFG×% dial  [3]

Where

QASC is th e quantity of anaesthetic supplied to the circuit;

FFG is the flow of fresh gas; and

% dial represents the concentration of anaesthetic at the output of theanaesthetic vaporiser.

In an equilibrium situation, the quantity of anaesthetic supplied to thecircuit (QASC) is equal to the total quantity of anaesthetic consumed(TQAC), thus equations [2] and [3] become equivalent, giving equation[4]:

(% IF−% EF)×V _(ALV) =FFG×% dial  [4]

where % IF, % EF, V_(ALV), FFG and % dial have the meanings indicatedhereinabove.

Equation [4] reflects the pharmacodynamic equilibrium of a closedcircuit when nothing is released to the outside. As can be appreciatedin equation [4], when the system is in equilibrium, the totalanaesthetic consumed (left part of equation [4]) is equal to the volumeof volatile anaesthetic supplied to the circuit (right part of equation[4]).

However, normally a loss of gas to the outside occurs, given that moreflow of fresh gas (oxygen) is always administered than that strictlycaptured by the patient. Thus, in actual fact, the anaesthetic suppliedcan be considered as equal to the sum of anaesthetic consumed by thepatient and the anaesthetic released to the outside.

Therefore, in a particular embodiment of the method object of thisinvention, the quantity of anaesthetic supplied to the circuit (QASC) isequal to the total quantity of anaesthetic consumed by the patient plusthe quantity of anaesthetic lost to the outside, which can thus beexpressed by equation [5]:

QASC=(% IF−% EF)×V _(ALV) +[FFG−((%O_(2 inh)−%O_(2 exh))×VR)]×%FE  [5](anaesthetic consumed) (anaesthetic lost)

where

QASC, % IF, % EF, V_(ALV), FFG and VR have the meanings indicatedhereinabove;

O_(2 inh) is the fraction of inhaled oxygen, supplied by the flow offresh gas; and

O_(2 exh) is the fraction of oxygen exhaled by the patient.

As can be appreciated from equation [5], the more the quantity of oxygensupplied by the fresh gas approaches the quantity of oxygen consumed bythe patient, the less the quantity of oxygen lost to the outside, andtherefore, the efficiency will be maximum.

The supply of the quantity of anaesthetic to the circuit is effected,according to the present invention, by means of opening the dial of theanaesthetic vaporiser. This constitutes a substantial difference to theclassical method of supplying anaesthetic to the circuit, which consistsof increasing the flow of fresh gas. Although this achieves a rapidincrease in the inhaled fraction, this effect is achieved at the expenseof losing through the valve for expulsion all the excess oxygencontaining an appreciable concentration of corresponding anaesthetic.The invention provides, in definitive fashion, a considerable saving ofanaesthetic because the FFG is never increased and anaesthetic is notlost to the outside. In contrast to that established normally, thepresent invention makes it clear that modifying the dial opening to varythe concentrations of anaesthetic (and not the flow of fresh gas) anexact, ecological and efficient model is achieved for the supply ofanaesthetic to the system.

Finally, the volatile anaesthetic supplied by the vaporiser is mixedwith the flow of fresh gas, which drags it along before being introducedinto the patient.

The method of the invention is not effected on a human or animal body asit is carried out at a stage prior to the introduction of theanaesthetic into the patient, it is suitable for quantifying,controlling and optimising the bases for a possible automation of aanaesthetic work station that functions with a low-flow circularcircuit.

The method of the invention has numerous advantages, which include thefollowing;

low consumption of anaesthetic

minimal loss of anaesthetic, because, as the patient does not consumemore than 300 ml of oxygen per minute, every time values much largerthan this figure are supplied to the system (which takes place when thecontrol of anaesthetic supply to the circuit is effected by increasingthe flow of fresh gas), the rest is lost to the outside with aconcentration of anaesthetic equal to that of the fraction exhaled bythe anaesthetised patient. Controlling the supply of anaestheticsupplied by varying the vaporiser dial, and without increasing the flowof fresh gas above 500 ml of oxygen, the loss of anaesthetic is minimal;

reduction in the emissions to the outside which implies (i) a reductionin the contamination of the operating theatre, contributing to betterhealth in the workplace, and (ii) a reduction in the environmentalcontamination not only as a result of the considerable reduction in theemission of chlorofluorinated compounds (CFC's) found in somehalogenated anaesthetics, which represent, approximately, 0.1% of thetotal release of CFC's to the atmosphere, thus contributing to apreservation of the ozone layer, but also because due to the fact thatthe anaesthetic mixture does not contain nitrogen protoxide, anitrogenated compound that contributes to the greenhouse effect iseliminated;

improvement in the temperature and humidification of the gas breathed inby the patient during anaesthesia, maintaining the values fortemperature (28-32° C.) and relative humidity (17-30 mg H₂O/l) which areclose to the optimal values, given that, as is well known, the higherthe flow rate of fresh gas the colder and drier the air breathed in bythe patient, which leads to a loss of warmth and a drying of thebronchial mucus; and

a contribution to the improvement of the monitoring process thanks tothe homogeneity and stability of the mixture of gases breathed in.

What is claimed is:
 1. A method of automatic control for the supply of avolatile anaesthetic to a closed anaesthetic circuit of low flow, whichcomprises the stage of mixing fresh gas and volatile anaesthetic beforeintroducing the mixture into the patient, the method comprising thesteps of: a) setting a ventilatory rate (VR) of the patient; b)automatically setting a flow of fresh gas (FFG) to 10% of the VR; c)determining a fraction inhaled (% IF) and exhaled (% EF) by the patientof volatile anaesthetic; d) setting a dial of an anaesthetic vaporiserto a value resulting from multiplying by 10 a differential of theinhaled fraction (% IF) and exhaled fraction (% EF) of volatileanaesthetic by the patient, wherein a concentration of anaesthetic at anoutput of the anaesthetic vaporizer (% dial) is equal to (% IF−% EF)×10,and changing, in a same proportion, the opening of said dial every timethat said differential of inhaled fraction (% IF) and exhaled fraction(% EF) of volatile anaesthetic by the patient varies; e) supplying aquantity of anaesthetic to a circuit (QASC) that covers at least a totalquantity of anaesthetic consumed by the patient (TQAC) by means ofopening of the dial of the anaesthetic vaporizer; and f) mixing thequantity of anaesthetic with the FFG before introducing said mixtureinto the patient.
 2. A method according to claim 1, wherein the TQAC isdetermined by the equation: TQAC=(%IF−%EF)×V _(ALV) where V_(ALV) is analveolar volume of ventilation.
 3. A method according to claim 1,wherein the QASC is determined by the equation QASC=FFG×% dial.
 4. Amethod according to claim 1, wherein the QASC is determined by theequation: QASC=(%IF−%EF)×V _(ALV)+[FFG−((%)O_(2 inh)−%O_(2 exh))×VR)]×%EF where V_(ALV) is an alveolarvolume of ventilation; %O_(2 inh) is a fraction of inhaled oxygen,supplied by the FFG; and %O_(2 exh) is a fraction of exhaled oxygen.